The invention concerns a brake pressure setting system for vehicle brakes including, for example, an antilocking system for stabilizing the dynamic behavior of a road vehicle by action upon its brake installation during brake pressure build-up, brake pressure retention and brake pressure reduction phases and wherein the wheel brakes of two driven vehicle wheels are combined in a static brake circuit and are connected via a pressure modulator and an electrically drivable inlet valve to an outlet pressure space of a main brake unit. The pressure modulator is designed as a stepped cylinder with two bore steps of different diameters and in which a stepped piston, with two flanges of correspondingly different diameters, is guided so that it can be displaced in a pressure-tight manner. These flanges of the stepped piston form axially movable boundaries of a modulation chamber and a control pressure space. The modulation chamber is bounded by the smaller diameter piston step and is connected via a pressure inlet control valve, in its basic position, to the outlet pressure space of the main brake unit as well as being permanently connected to a section of a main brake pipe of the static brake circuit which branches off to the wheel brakes. The pressure modulator is provided with a return spring which urges the modulator piston into its basic position that is associated with a maximum volume of the modulation chamber. The control pressure space, bounded by the larger piston step, can be alternatively connected, by an electrically drivable function control valve, to either a pressure outlet of a non-pressurized container of an auxiliary pressure source to displace the stepped piston in a first direction, or to a non-pressurized sump connection of said auxiliary pressure source. Connection to said pressure outlet displaces the stepped piston because of the pressure present in the control pressure space acting against the force of a return spring and the pressure in the modulation chamber, to its end position corresponding to a minimum volume of the modulation chamber and a maximum volume of the control pressure space. Electrically controllable brake pressure control valves are individually provided for each wheel brake and can be driven individually, or jointly, from a basic position, opening a connection between the particular wheel brake and the main brake pipe to permit brake pressure build-up and pressure reduction phases, into an alternative shut off position which is associated with the brake pressure retention phases. An electronic control unit is provided for generating signals required by the control system, for triggering the brake pressure control valves, the function control valve and the inlet control valve by processing output signals, characteristic of the motion behavior of the vehicle wheels from wheel rotational speed sensors.
Such a brake pressure setting device is, in association with an antilocking system for a road vehicle, the subject matter of the Applicants'own, older, German Patent Application No. P 37 23 875.2-21 which corresponds to U.S. patent application Ser. No. 07/220,168, filed July 18, 1988.
The antilocking system described in that application is intended for a road vehicle with front axle/rear axle brake circuit subdivision and real axle drive. The static brake circuit associated with the driven real wheels is connected to an outlet pressure space of a brake unit, designed as a tandem main cylinder of conventional construction, via a pressure modulator and an inlet valve which can be triggered electrically. The pressure modulator is designed as a stepped cylinder with two bores steps of different diameters, mutually offset by a housing step. A stepped piston with two flanges of correspondingly different diameters is guided in the stepped bores so that it is displaceable therein in pressure-tight manner. The two flanges forming axially movable boundaries of a modulation chamber and a control pressure space respectively. Fixed boundaries of the chamber and pressure space are each formed by an end wall of the stepped cylinder housing. In this connection, the modulation chamber is bounded by the smaller diameter piston step and is permanently connected to an initial section of the main brake pipe of the rear axle brake circuit which branches off towards the wheel brakes. In addition, the modulation chamber is connected by the inlet valve, in its basic position, to the brake unit secondary outlet pressure space associated with the rear axle brake circuit. The modulator piston is urged by a powerful return spring into the basic position associated with minimum volume of the modulation chamber. The control pressure space, bounded by the larger piston step, can be alternatively connected through an electrically drivable function control valve to a pressure outlet, or a non-pressurized tank, of an auxiliary pressure source. The stepped piston is then displaceable by outlet pressure of the auxiliary pressure source in the control space acting against the force of the return spring and the pressure present in the modulation chamber, to its end position corresponding to a minimum volume of the modulation chamber and a maximum volume of the control pressure space.
Individually electrically controllable brake pressure control valves are associated with the wheel brakes and these valves can be driven, individually or jointly, from a basic position opening the connection between the particular wheel brake(s) and the main brake pipe of the rear axle brake circuit to permit brake pressure build-up and brake pressure reduction phases, into an alternative shut off position, by which brake pressure retention phases can be achieved. An electronic control unit is provided which generates signals, required by the control system, for triggering the brake pressure control valves, the function control valve and the inlet valve by processing output signals, characteristic of the motion behavior of the vehicle wheels through wheel rotational speed sensors individually associated with the vehicle wheels.
In the antilocking system described in said German Patent Application No. P 37 23 875.2-21, the modulation chamber of the pressure modulator forms a variable volume section of the main brake pipe which, during "normal" braking operation (i.e. one not subject to an antilocking control system) is kept at the minimum value of its volume and, in the case of a braking operation subject to the control system, can be extended so that brake fluid can flow back into the modulation chamber from one of the wheel brakes subject to the control system. This provides for a reduction in brake pressure necessary for the antilocking control system at the wheel brake subject to the control system. Brake pressure restoration phases are controlled by the modulator piston being displaced in the direction of its position corresponding to a minimum volume of the modulation chamber.
Said German Patent Application No. P 37 23 875.2-21 does not reveal any sort of measures by which the brake pressure setting device described therein could also be used for a drive slip control system operating on the principle of retarding a vehicle wheel tending to spin by activating its wheel brake to such an extent that a stable dynamic behavior of the vehicle is achieved even during acceleration operation.
Such a drive slip control system could, however, be effected by providing a second pressure modulator for the brake circuit of the driven vehicle wheels. By this second pressure modulator, used for connecting the outlet pressure of an auxiliary pressure source to a control pressure space of this pressure modulator and shutting off its outlet pressure space from the main brake cylinder, brake pressure could be built up in a wheel brake of a vehicle wheel which shows a tendency to spin and which can be "selected" by the brake pressure control valve of the antilocking system.
Achieving a drive slip control system combined with an antilocking system in such a way would, however, be associated with substantial additional technical expenditure because, as well as the additional pressure modulator, further valves and triggering controls would be necessary.
The same applies in an analogous manner, to the combination of an antilocking system and a drive slip control system described in German Offenlegungsschrift DE No. 3,706,661 A1. In this combination, each of the vehicle wheels subject to an antilocking control system has its own pressure modulator. For each functional control, a functional control valve designed as a 3/3-way solenoid valve is provided, as well as a further pressure modulator and a drive slip control valve which are necessary to achieve the drive slip function. This drive slip function occurs through a control valve connecting or shutting off a control pressure space of this pressure modulator to, or from, the outlet of an auxiliary pressure source. The connection of the auxiliary pressure source to a control pressure space of the further pressure modulator, makes it possible to generate a pressure in the outlet pressure space which can be used as the brake pressure for the drive slip control. This pressure can be connected via inlet valves to the pressure modulators which can also be used for antilocking control.
German Patent DE No. 3,531,157 C1 also reveals a vehicle with all wheel drive and having antilocking control and drive slip control and wherein each of the wheel brakes can be used for one of the two types of control. Each wheel brake is associated with its own pressure modulator. In these pressure modulators, the inlet pressure space, which is connected to a pressure outlet of the brake unit, is sealed by the modulator piston against an outlet pressure space by which it is connected to the particular associated wheel brake through one solenoid valve for each wheel brake. For the antilocking control, a first drive pressure space is provided which can be connected by an antilocking control valve to an auxiliary pressure source. The application of pressure to the drive pressure space causes the modulator piston to be displaced, against the pressure generated by the brake unit and connected to the outlet pressure space in the direction of increasing the outlet pressure space connected to the wheel brake. For the antilocking control, a second drive pressure space is provided as part of each of the modulators. It is possible to connect this second drive pressure space to the auxiliary pressure source, or to shut it off therefrom by a drive slip control valve designed as a solenoid valve. Subjection of this second drive pressure space to pressure causes the modulator piston to experience a displacement in the direction of a build-up of brake pressure in its outlet pressure space. This permits activation of the connected wheel brake, as necessary for a drive slip control without actuation of the brake unit. The inlet, outlet and drive pressure spaces, of the pressure modulators of the known antilocking and drive slip systems are located adjacent to one another, within the particular modulator housing, viewed along the central longitudinal axis of the latter. This leads to a very "long" structural shape of the pressure modulators and to a corresponding disadvantage with respect to the installation space required. The mechanical construction of the pressure modulators is complicated on account of the numerous functional spaces, necessitated by the design of both the modulator housing and the modulator piston and is associated with high technical expenditure.
In addition, very high actuation forces are necessary in the case of a failure of the braking force amplifier of the brake installation, because the frictional resistances of numerous pressure modulator piston seals then have to be overcome by the pedal force with which the driver actuates the brake unit (which is all that is still available) before any worthwhile build-up of brake pressure can occur in the wheel brakes.
It is therefore the object of the invention to improve the known No. DEP-37,23,875.2-21 brake pressure setting device in such a way that it can be used for both the antilocking system and a drive slip control system operation and without any noticeable extra technical expenditure.
The invention achieves this object, when spin tendency appears on one of the driven vehicle wheels, by having the electronic control unit generate output signals by which the brake pressure control valves of the driven vehicle wheels are driven into their shut off positions, maintaining pressure at the brakes and the function control valve is driven into its function positions causing pressure relief of the control pressure space of the pressure modulator. This occurs when the lower limiting values .lambda..sub.A1 and/or b.sub.A1 of the drive slip .lambda..sub.A and/or the wheel peripheral acceleration b.sub.A are exceeded. These lower limiting values are lower than threshold values (.lambda..sub.A2 and/or b.sub.A2) beyond which the use of a drive slip control system becomes necessary. At the latest, when higher response threshold values .lambda..sub.A2 and/or b.sub.A2 are reached, the
A2 electronic control unit generates a first output signal for switching the pressure inlet control valve into its shut off position; a second output signal which initiates the switching back that one of the brake pressure control valves, associated with the wheel with the tendency to spin, into its basic positions and a third signal which initiates the switching back of the function control valve into its basic position, causing the control pressure space to be connected to the pressure outlet of the auxiliary pressure source. After the spin tendency has decayed, the electronic control unit generates that combination of output signals which causes all of the valves to be switched back into their basic positions.
By this, the electronic control unit is "extended" by a drive slip control function part, which controls the following functions:
When a spin tendency appears on one of the driven vehicle wheels, which is "recognized" by monitoring the drive slip .lambda..sub.A and the wheel peripheral accelerations b.sub.A, output signals are generated by the electronic control unit even before relevant response threshold values .lambda..sub.AS and/or b.sub.AS are reached which,
AS if they are exceeded, demand activation of the wheel brake of the vehicle wheel tending to spin, i.e. response from the drive slip control system. By these output signals, the brake pressure control valves of the driven wheels are driven into their shut off position and the function control valve is driven into its functional position causing pressure relief of the control pressure space of the pressure modulator. By this, the modulator piston experiences, due to the effect of its return spring, a displacement in the direction of increasing the volume of the modulation chamber, into which brake fluid can flow from the outlet pressure space of the brake unit via the inlet valve which is still in its basic position. Make-up brake fluid will then flow from the reservoir of the brake installation into the outlet pressure space of the brake unit. As soon as the modulation chamber has accepted a quantity of brake fluid, whose displacement into the brake circuit of the driven vehicle wheels can generate a sufficiently high brake pressure for the braking of these vehicle wheels, the inlet valve connected between the modulation chamber and the outlet pressure space of the brake unit, is switched into its shut off position by an output signal of the electronic control unit.
The brake pressure setting device is now prepared for drive slip control operation whose introductory brake pressure build-up phase on a vehicle wheel tending to spin and/or wheel acceleration of this wheel exceed the response threshold value .lambda..sub.AS and/or b.sub.AS. This occurs by switching the brake pressure control valve of this vehicle wheel back again into its basic through flow position and by also switching the functional control valve back into its basic position, in which the control pressure space of the pressure modulator is again connected to the high pressure outlet of the auxiliary pressure source and, in consequence thereof, the modulator piston experiences a displacement in the direction of reducing the volume of the modulation chamber. Because of this, brake fluid is forced into the wheel brake of the vehicle wheel subject to the control system and brake pressure is built up in this wheel brake to retard this vehicle wheel. A brake pressure retention phase on the vehicle wheel subject to the control system can be controlled by again switching its brake pressure control valve into its shut off position.
In order to control brake pressure reduction phases of the drive slip control system, it is particularly advantageous if when the spin tendency decays, the electronic control unit first generates an output signal by which the function control valve is driven back into its actuated position causing pressure relief of the control pressure space of the pressure modulator so that, initially, brake fluid is again accepted by the modulation chamber of the pressure modulator from the wheel brake of the vehicle wheel subject to the control system and the inlet valve is only subsequently switched back into its basic position connecting the modulation chamber with the outlet pressure space of the brake unit. By this operation the appearance of "pressure shocks" in the brake unit, which can lead to undesirable load on the piston seals forming the boundaries of the outlet pressure space, can be substantially reduce.
In any event, all the valves are switched back into their basic position, after the decay of the spin tendency, in order to terminate the drive slip control.
The preparation of the brake pressure setting device for drive slip control operation by having the electronic control unit generate a signal causing the switching of the function control valve into its position connecting the control pressure space of the pressure modulator to the high pressure outlet of the auxiliary pressure source immediately after the pressure inlet control valve is switched into its shut off position, has the advantage that the brake fluid in the modulation chamber and the main brake pipe of the brake circuit of the driven vehicle wheels connected to it, is already under high pressure, when the brake pressure control valve of the wheel brake of the vehicle wheel to be subjected to the control system, for the purpose of brake pressure build-up, is switched back again into its basic position. This provides for a rapid response of the drive slip control.
A preferred arrangement of the brake pressure setting device according to the invention, provides for a displacement indicator which emits electrical output signals characteristic of the position of the piston of the pressure modulator.
By such a displacement indicator, it is, for example, possible to very accurately direct a withdrawal motion of the modulator piston (which causes the brake pressure reduction in a wheel brake) to that position from which (when the pressure build-up phase commences) displacement of the modulator piston takes place in the direction to build-up of brake pressure. This makes it possible to reduce to a minimum, or even to avoid, balance flows between the brake unit and the modulation chamber, which might otherwise occur when the inlet valve is open. This has the object of avoiding pressure shocks in the brake unit. In addition, the output signals of such a displacement indicator can also be used in an advantageous manner for both the drive slip control operation and the antilocking control operation in order to achieve proper control phases to meet the requirements of the apportionment and/or reduction of pressure. Thus an improvement in the control behavior for both types of control is achievable in an obvious manner.
The same applies in an analogous manner to the design of the brake pressure setting device which has a force signal generator integrated in the modulator piston. The electrical output signal of this signal generator is a direct measure of the brake pressure in the wheel brake, subject to the control system, and can therefore also be used for correct brake pressure control to suit the control requirements.
Although the brake pressure setting device is fail-safe to such an extent that, should the electronic control fail, the inlet valve, the brake pressure control valves and the function control valve return to their basic positions in which normal brake operation is possible, it may nevertheless be expedient if, as an additional safety measure, a bypass flow path is provided in parallel to the brake pressure setting device. This includes a bypass pipe directly connected to the outlet pressure space of the brake unit associated with the brake circuit of the driven vehicle wheels. This bypass pipe can be shut off to the outlet pressure space by an outlet control valve. This outlet control valve is connected to each of the vehicle wheel brakes of the driven vehicle by an outlet non-return valve. This outlet control valve is designed as a solenoid valve which can be driven into a through flow and shut off positions co-jointly with the inlet control valve. Thus, should the inlet valve "stick" in its shut off position after an antilocking or drive slip control phase, at least the brake pressure can be reduced again by withdrawal of the brake pedal.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.